Acoustic horn



Feb. 17, 1931. H, B, ELY 1,792,634

ACOUSTIC HORN Filed Jan. 21. 1929 3 Sheets-Sheet 1 l i Q Q 1 l* D o q 1,/ Q I j f 4 0K I 1 l n h 111 11:: 1 o 4\- VIT I '-1 :it I

Tief-44 akten/w11 Feb. 17, 1931. H. B. ELY 1,792,634

ACOUST I C HORN Filed Jan. 21. 1929 3 Sheets-Sheet 2 l gnventot l Hiram-LELQy anonwq H. B. ELY

ACOUSTIC HORN Feb. 17, 19.31.

Filed Jan. 3 Sheets-Sheet 3 3mm o@ Hiram E Eiy alito: new

. horn especially adapted for a sound locating Patented Feb. 17, 1931 UNITED STATES PATENT OFFICE .ACOUSTICv HORN Application filed January 21, 1929.. Serial No. 333,927.

(GRANTED UNDER THE ACT (1F MARCH 3, 1883, AS AMENDED APRIL 30, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for the Govern- 'ment for governmental purposes `without the payment' to me of any royalty thereon.

The subject of this invention is an acoustic apparatus.

The range at which a sound locatingvapparatus is capable of effectively being employed depends upon the intensity of the auditory impression at the small end of the horn and this is governed by the amount of sound energy at the small end compared to the amount of energy enterinor the mouth of the horn. Sound waves bei pressure waves increase in forceas they proceed towards the small end of the horn until a certain critical value is reached after which the vincrease in friction due to attenuation of the horn more than offsets the increase in intensity and the energy diminishes. Accordingly the shape and length of the horn andthe dimensions of its small and large ends are factors entering into a determination of its eiciency.

The shape ofthe most eliicient type of horn known at present is 'that in which the crosssectional area' is an exponential function of Athe distance from the small end of the horn. But in order to obtain thebest results the rate of change of cross-sectionalarea cannot be selected at random but must bedetermined by a 4prior consideration of the other dimensional factors. I have discovered that the rate of change shouldpossess suchl a value that the length of the horn is substantially equalto the wave length of the lowest frequency of sound to be transmitted through` out the horn. -With the dimensions of the small and large endsof the horn and the optimum angle `of the large end selected, then cured. This invention, therefore, relates also to the dimensioning of a receiver horn.

` Where in order to secure the designated length of a horn it becomes necessary to bend it for convenience of mounting the curvature of the elbows and their positioning with rc-A spect to the length of the horn plays an important rle in transmitting sound waves without interference and in preserving phase relations., Sound waves on entering a horn from the free 'atmosphere pass through a transitory period of turbulence before the formation o'the confined wavetakes place. Further, where the large end of the horn is rectangular for thev purpose of obtaining directively the point of transition to curvilinear 'form must be properly located withA respect -to the length of the horn to avoid non-uniform disturbances.

In arranging horns in pairs for thebinaural comparison of sound no attention has heretofore been given to a vspacing'of the horns to produce a maximum phase difference. Theoretically the ater the separation betweenhorns, thev s arper will be the 7 phase effect but I have found `in application that this holds true only up to a certain point beyond Which incorrect interpretations or impressions are made. The mostv effective results are obtained when the separation of the .horn axes is equal to the wave length of the .lowest pitch to be received.

With the foregoing and other objects in View, the invention resides in the novel arrangement and combination of parts and in the details of construction hereinafter de scribed and claimed, it being understood that changes in the precise embodiment of the in vention herein disclosed may be made within thescope of what is claimed Without departingl from the spirit of the invention.

. A practical embodiment of the inventlpn is illustrated in the accompanying drawings, wherein:

` Fig 1 is 'a view inrear elevation of a sound locating apparatus equippedv with the 1mproved horns;

Fig. 2v is a view in side elevation thereof;

` Fig. 3 is a sectional view of one of the horns; f

l of the trumpet type comprising four horns rigidly interconnected i'n parallel and arranged in two combinations on intersecting base lines, one such combination comprising the horns A-A for determinin azimuth, and another comprising the horns -B for determining elevation. The support sections 5 of the horns A- -A are trunnioned in spaced standards 6 secured to a rotatable ca'rra e 7 and the support sections of the horns B are'connected 'to the support sections 5 by means of yokes 9. The interconnected horns are rotated in unison about the horizontal axis of the horns A-A by means of an elevating niechanisin 10.

. The mounting is so arranged that the distance between the axes of reception of opposite horns is not greater than the length of the longest sound waves it is desired to locate. This selection of the separating distance is based on the fact that accurate binaural comparison is only possible when the sound waves are received less than 180 out ofv phase or in other words when the axis of the sound locatin apparatus is pointed very nearly towards t e sonrea, of sound, If the two horns were separated a distance greater than that specified, ,the listener might receive adjacent sound waves in such phase relation as to mislead him -to believe he was receiving the saine wave in his two ears, less than 180--out of phase thus resulting in incorrect pointing of the ap aratus.

The horns are substantlally alike and, as secn in Figs.`1, 2 and 3, are' made in four sections, namely: a mouth section 11a reverse curve section 12 paralleling the mouth section, the support section 5 for the horn A and"v 9 for the horn B extending perpendicularly to the curved section and a flexible tube 5 or 8. The sections 11, 12 and 13 are rigid units composed of aluminum of sutlicient thickness and density to prevent an absorption of more than 5% of any sound which impingcs uponthcm and of suflicient rigidity to prevent vibratory action by sound energy. The flexible tube section 13 may be extended through a portion 14 of uniform diameter throughout its length for convenience'in attachment to a head set 14a. Y

The horns are each of exponential crosssection, the form of the equation used in determining the design of: the horn being A=Aoem in which A is the cross-sectional area at any point; A0 is the cross-sectional area at the small end; e is the Naperian base; M is a constant determining the' rate of flare;

' is the distance of A along the axis incasured from the small end,-providing m is given such a value as to make where L--the wave length of the lowest frequency to be obtained,`K-a constant ,whose value has been found to lie between 3 and 4 for best performance, when the total length of th'e horn axis lies between %L and L.

A practical horn which gives excellent per'- ormance in the reception of sound emanating from an airplane havinga fre uency between 75 and 150 cycles per secon possesses the following dimensions. The total lengthof the horn is 126 inches divided as followsthe mouth section 48 inches, the reverse curve section, the support section and the flexible section, each 26 inches.

The selection of the diameter of the small end having beendetermined' after experiments to lie between .625 and 1 inch, the diameter of the large end having likewise been determined to produce the bestresults when it is approximately one-fourth of the horn length or 30 inches, and where the constant m has a value of .03 for inch units of length, the corresponding increase in cross-sectional area for every two inches will be approximately 6% in order to obtain a horn of the prescribed length and prescribed diameter of the large en The constant m may have a value between .03 and .07.

.A horn constructed in accordance with the foregoing values will give to the walls of the mouth of the horn an angle of approximately 30 and .this angle has been found t0 afford the best reception while giving a suiicient field.

.In bending or curving a horn of great length to reduce its overall dimensions considerable distortion and loss of energy are introduced by the elbows due to the redections which take place and cause interference of one sound wave with another. I have found by experiment that this condition is reduced to la minimum by maintaining a strai ht large end section for at least onethir of the total horn length in order to allow the sound to pass the transitory period of turbulence occasioned by entering the horn from the free atmosphere and conform itself to the gradual attenuation of the horn.

The radius of the first elbow 15 should therefore be not less than the dimension of the curve and attached to the large end section to supply rigidity thereto.

The foregoing considerations enter into the design of a horn irrespective of-the shape of the horn in cross-section. Generally acoustic horns are circular in cross-section but the particular problem presented in the location of distant sounds renders it advisa'je to employ a structure or arrangement which will increase the directivity of the horns by bafiiing non-frontal sound waves.

Referring particularly to Fig. l, the mouth section 11 of the horn is rectangular in crosssection, the inner side of each horn when arranged in pairs being constituted by a lane surface 17 and remaining sides being ared to produce reflections that will aid in the location of sound. Of the three flared walls the one 18 opposite the plane wall 17 has a more pronounced curvature which acts in the off horn C of Fig. 4, either to baille nonfrontal sound waves indicated by the arrows J-b and by reflection cause them to pass out of the horn or as indicated by the sound waves c-c to effect their intensity and time of arrival.

The same curvature serves in the near horn D of the illustration to increase its perception and augment the binaural phase effect. The sound waves a-a approachlng the horn in a plane perpendicular to the frontal direction will enter both horns and be transmitted in phase.

From the foregoing it is seen that the plane surface 17 plays an important part in establishing an impression of lateral direction by creating a maximum phase difference and a variation in intensity. Such an impression is more pronounced and is more effective when the field of recept-ion with both horns is limited. This is accomplished by converging the plane walls of opposite horns, the inclination or deviation from parallelism of each horn being substantially 5. For customary ranges this provision will give an exclusion in the ofi horn for sounds arriving greater than 5 from the center line of the two horns. In Fig. 4 the sound wave b is shown as being excluded from the horn Whereas it would be directed into the horn if the plane surface were not at an inclination.

Where the mouth section of the horn is rectangular in cross-section for the reasons set forth the transition to curvilinear form should not occur in the straight mouth section but rather at a point in the reverse curve section beyond the elbow, which point is approximately at t-he center of the tota-l horn length.

I claim: l

1. A receiver horn having an exponential variation of cross section with the length and an internal diameter at the small end between .625 and one inch.

2. A receiver horn, the area of whose cross variation of cross section with the length and i an internal diameter at the small end more than two per cent of the internal diameter at the large end.

4. A receiver horn, the area of the cross section of which is an exponential function of the distance from the small end, the diameter of the small end being greater than one-half of one per cent of the length of the horn, and the length of the horn being approximately equal to the wave length of the lowest frequency of sound to be transmitted.

5. A receiver horn, the area of the cross section of which is an exponential function of the distance from the small end and the diameter of the small end being greater than one-half of one per cent of the length of the horn.

6. A receiver horn, the area of the-cross section of which is an exponential function of the distance from the small end and the length of the horn being approximately equal to the wave length of the lowest frequency of souno to be transmitted.

7. A receiver horn having a cross section which is an exponential function of the distance from the small end and having a mouth whose walls make an angle of approximately thirty degrees with the axis of the horn.

8. A receiver horn, the area ofthe cross section of which is an exponential function of the distance from the small end and the diameter of the large end being approximately twenty-five per cent of the length of the horn.

9. A receiver horn having a cross sectional area varying according to the equation A=Aoemf where m is given such a value as to make L being the wave length of the lowest fre` quency to be received and K being a constant whose value lies between 3 and 4.

10. In a horn having an exponential variaof parallel spaced horns mounted for movement in unison, the inner sides of the two horns converging at an angle of ap roximately five degrees to a horn axis an the opposite side of each horn being flared.

12. In a sound receivin apparatus a pair of parallel horns having teir axes separated by a. distance not greater than the length of the longest sound wave to be received.

` HIRAM B. ELY. 

